The disclosures herein relate generally to computer systems and more particularly to redundant forced air cooling in a computer chassis.
Redundant forced air cooling in electronics is generally handled in one of two ways. Assuming the cooling mechanism is forced air movement, redundancy can be handled by a serial implementation of the air movers (fans) or a parallel implementation. For the parallel implementation specifically, it is common to use an "N+1" strategy; meaning that there is one more fan than is actually needed to remove heat from the enclosure. If one of the fans fails, the remaining fan or fans move sufficient enough air to cool all components within their specified limits. A disadvantage of this strategy is that if any of the fans are proximally located to critical components, and are providing an increased air velocity to maintain proper temperature, then a failure of that fan might cause specific temperature increase unless a second fan were located close enough as well. Assuming the definition of successful redundant N+1 parallel cooling, means that the failure of any one fan does not offer any proximal risk to any component, then the following additional problem exists.
In a parallel combination of fans, the failure of one fan likely causes an "air short circuit". The pressure upstream of a fan is lower than the pressure downstream of a fan. In a system where fans blow into an enclosure, the pressure in the enclosure is greater than externally. In a system where fans draw out of the enclosure, the pressure is less inside the enclosure than externally. In either case, a failed parallel fan provides an "open window" for a higher pressure air to flow back to the lower pressure space (an air short circuit). The failure of one fan therefore, instead of reducing proportionally one fan's contribution of the total air flow, could possibly reduce the total air flow by more than the contribution of one fan. Because of this, the cooling system has to be "over-designed" to take into account this backward flow or to prevent it entirely.
In the past, various attempts have been made to improve redundant cooling systems. In U.S. Pat. No. 5,210,680, a card cage is provided with an air cooling system employing deformable baffles in the air flow path to the spaces between the printed circuit cards. The baffles are deformed upon assembly in order to adapt to the thermal conditions to be expected during operation of the card cage. The baffles provide minimum openings which produce a pressure in front of the openings which effects a more uniform flow distribution via the openings, and which also permits an increase in the air flow rate to the printed circuit cards which are subject to strong thermal stresses.
U.S. Pat. No. 5,572,403 discloses a cooling subsystem and method, for a chassis of a computer system. The cooling subsystem comprises: (1) first and second cooling fans having first and second motors for driving the first and second cooling fans, respectively, and (2) a common plenum substantially shrouding and providing a pathway for air communication between the first and second cooling fans. The first and second fans cooperate to provide an optimum rate of air flow from without the chassis to within the chassis to provide air exchange within the chassis. The air flow within the chassis is in a predetermined direction to provide directed cooling of a specified device within the chassis. The common plenum allows the first and second fans to continue to cooperate to provide a minimum air flow and air exchange within the chassis. The air flow remains in the predetermined direction to continue the directed cooling of the specified device when one of the first and second motors fails. The plenum comprises a bypass aperture for allowing the air to alternatively enter and exit the plenum. The bypass aperture increases a rate of air flow across the subsystem when one of the motors fails.
U.S. Pat. No. 5,793,610 discloses a cooling fan system for a chassis configured to contain heat generating electrical components and to assume horizontal and vertical alternative operating orientations. The cooling fan system includes a support member within the chassis and having an air flow opening formed therein. Additionally, the cooling fan system includes a louver member attached to the support member to rotate between an open position, wherein a substantial air flow is allowed through the air flow opening, and a closed position wherein the louver member covers the air flow opening. The louver member may be made from plastic, however, other types of rigid materials that can withstand the chassis' operating conditions could also be used, such as metal. The axis of rotation of the louver member is oriented with respect to the chassis to allow gravity to urge the louver member toward the closed position when the chassis is positioned in either the horizontal or vertical operating orientations, to prevent a back-flow of air in the chassis.
The diagonal nature of the plate mountings permits the device to work in the "tower" mode or the "rack" mode, i.e. within these two 90.degree. extremes. During normal (non-failed) operation, each fan's airflow is impeded and has to overcome the weight of the rigid louvers in order to keep the passageways open for airflow. Thus, the weight of the plastic louver acts against the airflow.
In U.S. Pat. No. 5,787,971, a cooling device for an electrical component, such as a computer processor, includes a heat sink and a plurality of fans. The fans are redundant in that sufficient cooling is supplied should one fan fail. The fans are independently replaceable in situ without power loss to the other fan or the processor. The heat sink includes a base plate having a bottom for surface contact with the processor, a front row of fins projecting upward from the front of the base plate and a rear row of fins projecting upward from the rear of the base plate. A plurality of fans are mounted within a cavity between fan rows. Each fan blows a flow of air such that the flow impinges on the base plate. A central channel between the front and rear fin rows provides a path for air such that should one fan fail, the other fans remain operable for generating a flow of air that impinges on the top surface beneath the failed fan for cooling.
Therefore, what is needed is a back flow limiting apparatus for a failed fan in a parallel redundant fan cooling system which is economical, easy to install and/or replace, and non-complex.